Radio bomb release system



Feb. 18 1947. R, c. SANDERS, JR

RADIO BOMB RELEASE SYSTEM Filed April 20, 1944 2 Sheets-Sheet 1 $5 b gko m h IN VEN TOR.

Feb. 1947. R c SANDERS, JR V RADIO BOMB RELEASE SYSTEM Filed April 20, 1944 2 Sheets-Sheet 2 APP/WE m arrme/mr/rrzoa m1 at I? FLECTEO ig/v44 50? 1% grr r r Z 5 7/ V -29 I M Patented Feb. 18, 1947 RADIO BOMB RELEASE, SYSTEM Royden C. Sanders, J r., Hightstown, N. J assignor to Radio Corporation of America, a corporation of Delaware Application April 20, 1944, Serial No. 531,953

9 Claims.

This invention relates to automatic bomb release systems, and more particularly to the art of releasing bombs from aircraft in response to radio measurements of the distance of a target and the speed of the craft with respect to the target. In my copending U. S. patent applications Serial Numbers 524,794, 524,795 and 524,796, filed on March 2, 1944, and entitled Radio bomb release systems, which applications are assigned to the same assignee as the instant application, I have described means for continuously measuring the distance of a target and the speed of the craft with respect thereto, and for releasing a bomb upon the occurrence of the predetermined relationship between these quantities which is required for a hit.

It is the principal object of the present invention to provide improved methods of and means for automatic bomb release, wherein the time of release is continuously predicted.

Another object is to provide improved methods of and means for automatic bomb release Wherein continuous measurement of speed and distance up to the instant of release is not required.

A further object of this invention is to provide, in a radio bomb release system of the described type methods of and means for remembering the instant of release, whereby the time interval to elapse before release is continuously computed notwithstanding momentary failure of the signals employed for measurement of speed and distance.

Still another object is to provide, in a system of the described type, methods of and means for affording a warning that release is to occur at the end of a predetermined time interval.

These and other objects will become apparent to those skilled in the art upon consideration of the followin description with reference to the accompanying drawings, of which Figure 1 is a schematic diagram of an embodiment of the invention, Figure 2 is a graph illustrating variations in frequency of signals transmitted and received in the operation of the system of Figure 1, Figures 3 and 4 are graphs of square wave switching voltages occurring in the operation of the system of Figure 1, and Figure 5 is a graph illustrating switched counter output currents produced in the operation of the system of Figure 1.

In the practice of the present invention, it is proposed to employ a continuously running clock, or its equivalent, to operate a circuit closer for actuating the bomb release mechanism. The clock is set back from its circuit closing position by an amount corresponding to the velocity of the aircraft divided by the distance from the target, at the instant of setting. Thus the clock will indicate continuously the time from target, of period which will elapse before the target is reached. By means of radio reflection speed and distance measuring equipment of the type described in the above-mentioned copending applications, the clock setting may be made automatically and checked continuously, with automatic correction in response to variations in the speed of the craft with respect to the target.

Refer to Figure 1. A radio transmitter l is connected to an antenna 3 and to a frequency modu lator 5. A switch 2 is connected in the anode supply circuit of the transmitter I, which includes a battery 4. The modulator 5 may be of the vibratory variable capacitor type such as that described in copending U. S. patent application Serial Number 471,003, filed January 1, 1943, by S. V. Perry, and entitled Capacity modulator unit, or any other known device for varying the frequency of the transmitter I in response to a modulating voltage. The input circuit of the modulator 5 is connected to a wave shaping circuit l which is connected through a voltage divider 9 to a battery II and a periodic switch 13. The switch [3 is arranged to be operated by a cam l5 driven by a'motor IT. The motor I! is connected through a switch I9 to a power source such as a battery 2|.

A receiving antenna 23, similar in construction to the antenna 3, is connected to a detector 25. The transmitter I is also connected to the detector 25 through a line 21. Both antennas 3 and 23 are preferably directive, and are arranged to provide maximum response in the same direction.

The output circuit of the detector 25 is connected to an amplifier 29, which is provided with an A. V. C. circuit including a rectifier 30 connected to rectify a portion of the amplifier output and apply the rectified voltage to a bias circuit of the amplifier in any known or conventional manner for controlling the amplifier gain. The output of the rectifier 30 is also applied to a D.C. amplifier 32, which is connected to a relay 34. A D.C. source comprising a battery 36 and a voltage divider 38 is connected to the amplifier 32 and adjusted to bias it so that the relay 34 is open in the absence of output from the amplifier 29,- and closed when the output of the amplifier 29 exceeds a predetermined value. The output circuit of the amplifier 29 is connected to an amplitude limiter 3|. The output circuit of the 3 limiter 3| is connected to a pair of averaging cycle counter circuits, generally designated by the reference numerals 33 and 35, respectively.

The counters 33 and 35 are provided with a common load resistor 37, which is connected to the control grid of an amplifier tube 39. The anode of the tube 39 is connected directly to a D.-C. source 4|. The cathode circuit of the tube 39 includes a resistor 43 tapped at a point 44. The cathode of the tube 39 is connected to the cathode of a tube 46. The control grid of the tube 46 is connected to a bias source including a battery 53 and a voltage divider 50. A pair of relays 45 and 41 are included in the anode circuit of the tube 46.

The counter 33 includes a. capacitor 49 connected to the cathode of a diode and to the anode of a triode 53. The control grid of the triode 53 is coupled to the switch I3. The cathode of the tube 53 is connected to the load resister 31. The anode of the diode 5! is connected to the cathode circuit of the tube 39 at the point M on the resistor 43. The counter 35 comprises a capacitor 55 connected to the cathode of a diode 5i and to the anode of a triode 59. The control grid of the triode 59 is coupled through a. phase inverter circuit 6| to the switch I 3. The anode of the diode 51 is connected to the load resistor 31. The cathode of the triode 59 is connected to the cathode of the tube 39. The lower end of the resistor 31 is connected to a; voltage divider 63, across a battery 35. The load resistor 31 is bypassed to ground by a capacitor 67.

The relays 45 and 41 are each provided with single pole double throw contact arrangements, connecting a reversible motor 69 to a battery H, through the contacts of the relay 34. The relays 45 and 41 are adjusted so that the drop out current of the relay 45 just slightly exceeds the pick up current of the relay 4?. Thus when the current flowing through. the windings of the relays 45 and 41 is of a magnitude between that required to pick up the armature of the relay 41 and: that which will allow the armature of the relay &5 to drop, the armature of the motor 69 is disconnected from the battery I I. When the current is less than. that required to pick. up the relay 41 current flows through the armature of the motor 69 in the direction indicated by the solid arrow, causing the motor to rotate in, for example, a clockwise direction. When. the current is sufficient to pick up the armature of the relay 45, current flows through the armature of the motor 69 in the direction indicated by the dash arrow, causing rotation in the reverse direction. If the received signal fails, the relay 34 is energized, disconnecting the battery H from the circuit.

The operation of the system thus far described is as follows: The motor ll operates the switch l3 by means of the cam I5 to connect the battery H periodically to the voltage divider 9, thus producing a square wave voltage variation of the voltage across the voltage divider 9. The square wave voltage is attenuated to an extent depending upon the position of the adjustable tap of the voltage divider 9, and is applied to the wave shaping circuit 1. The wave shaping circuit 1- includes a low pass filter or other means for integrating the square wave input with respect to time to produce an output of triangular wave shape. The triangular wave output of the wave shaping circuit 1 is applied to the modulator 5 causing corresponding triangular wave variation of the frequency of operation of the transmitter l. The frequency modulated signal produced by the transmitter I is radiated by the antenna 3 to the target, not shown. Part of the energy striking the target is reflected to the receiving antenna 23. The received reflected signal is combined in the detector 25 with some of the original frequency modulated signal, which is conducted directly from the transmitter l to the detector 25 through the line 21. The output of the detector 25 includes a beat signal having a frequency equal to the difference between the instantaneous frequency of the transmitted and received signals. The beat output of the detector 25 is amplified by the amplifier 29 and limited to a constant amplitude by the limiter 3|. The output of the limiter 3! is a square wave voltage having a frequency equal to the difference between the frequency of the transmitted and receivedsignals and a constant amplitude, Es.

Referring to Figure 2, the frequency of the transmitted signal is represented by the solid line 13. This frequency varies throughout the modulation cycle in response to the triangular wave output of the wave shaping circuit 1, between upper and lower limits f2 and f1, respec-- tively, about a mean value in. The sweep width f2--f1 is proportional to the amplitude of the triangular wave input to the modulator 5, and hence is a function of the position of the adjustable contact of the voltage divider 9. The reflected signal is delayed with respect to the transmitted signal by the time required for the radiation to travel from the transmitting antenna 3 to the target, and back to the receiving antenna 23. This is indicated by the dotted line E5 in Figure 2. The reflected signal varies in frequency over the same range 12-41 as the transmitted signal, but constantly differs in frequency from the transmitted signal by an amount proportional to the distance. This difference in frequency is Sf d 246 cycles per second, where S=,f2f1 in megacycles per second, in is the modulation frequency in cycles per second, or frequency of operation of the switch [3, and d is the distance in feet. If the equipment is moving toward the target, the received signal is increased in frequency, owing to Doppler effect, by an amount 246 6' cycles per second during increase in frequency of the transmitted signal, and

fm 2 n 246 C cycles per second. during decreaseln frequency of the transmitted signal.

The constant amplitude beat frequency output ofthe limiter 3| is applied to both of the counters 33 and 35. During the modulation upsweep, or increase in frequency of the transmitted signal, the switch I3 is closed, applying a positive pulse to the control grid of the triode 53 and to the phase inverter El, as indicated by the graph of Figure 3. The phase inverter BI provides a negative pulse which is applied to the control grid of the triode 59 of the counter 35, as indicated by the graph of Figure 4. The triode 59 is thereby cut off, and the counter 35 prevented from operating. The triode 53 is conductive, allowing the capacitor 49 to charge through the resistor 31 during positive half cycles of the output of the limiter 3|. During negative half cycles of the limiter output, the capacitor 49 is discharged through the diode 5| to the potential appearing at the lower end of the resistor 43, which is slightly less than the potential at the cathode of the tube 39 and hence the cathode of the tube 53. The values of the capacitor 99 and resistor 31 are such that the capacitor 69 becomes substantially fully charged to the limiter output voltage ES during each cycle of the limiter output. Thus during the modulation upsweep, the counter 33 causes a current in to flow downward through the resistor 31, as indicated by the solid arrow. This current is proportional to the product of the charge deposited in the capacitor 49 during each cycle, and the number of cycles per second:

where fu is the beat frequency, Q is the charge per cycle, C1 is the capacitance of the capacitor 49, and Es is the amplitude of the output of the limiter 3|. Since During the modulation downsweep, the switch I3 is open, providing a negative pulse at the grid of the triode 53 and the phase inverter 6i, and a positive pulse at the grid of the triode 59, as indicated by the portions 19 and BI, respectively, of the graphs of Figures 3 and 4. The counter 33 is now inoperative and the triode 59 is conductive. During negative half cycles of the limiter output, the capacitor 55 is charged through the diode 51 and the resistor 31. During positive half cycles, the capacitor 55 is discharged through the triode 59 to the potential existing at the cathode of the tube 39, which is substantially equal to that at the anode of the diode '51. Thus during the modulation downsweep, the counter 35 causes an average currentid to flow upward through the resistor 31 as indicated by the dash arrow. This current is proportional to the product of the charge deposited in the capacitor 55 during each cycle, and the W 05 number of cycles per second:

id=frQ=frc2Es d The average current through the resistor 31 is ll 1z d Refer to Figure 5, wherein I1 is the average component of current during upsweep due to distance, I2 is the average component of current during downsweep due to distance, I3 is the resultant average component of current due to distance, I4 is the increase in negative average component of current during downsweep due to speed, and I5 is the decrease in positive average current during upsweep due to speed. The resultant average voltage across the resistor 31 is where R is the resistance of the resistor 31.

The tap of the voltage divider 63 is adjusted to apply a positive potential of, for example, approximately 70 volts to the lower end of the resistor 37. The purpose of this arrangement is to provide a suitable operating point for the cathode follower tube 39. Denoting this voltage as 61, the total voltage at the control grid of the tube 39, referred to ground potential, is eo+ei. Inasmuch as the entire load of the tube 39 is in the cathode circuit, th anode current will assume a value such that the drop in said load circuit is very slightly greater than the voltage between the control grid and ground, and as a practical matter, substantially equal to the grid voltage. Thus the anode current i of the tube 39 is substantially proportional to the voltage eo+e1.

The relays 45 and 41, voltage divider 50 and the bias voltage e1, are adjusted so that the motor 69 is disconnected from the battery 1|, as described above, when 60:0. Under this condition,

and

This may be rewritten as:

is the distance to the target divided by the speed of approach to the target, and is therefore the time T which will be required to reach the target. In the present example, the quantities im, in, C1 and C2 remain constant, while S may be varied by means of the voltage divider 9. Accordingly,

where Thus it is apparent that the sweep width S is inversely proportional to the time from target, T, and the position to which the voltage divider 9 must be adjusted to prevent the motor 69 from running is a measure of the time T. The value of 7c, the proportionality constant between the reciprocal of the sweep width and the time T from target, may be predetermined in accordance with practical considerations by proper choice I of the constants in in, C1 and other suitable units of time from target.

The voltage divider 9 is constructed to provide a reciprocal attenuation vs. rotation characteristic, i. e., the resistance is tapered so that the sweep width 8 is inversely proportional, over the operating range of the equipment, to the angular position of the adjustable tap, as indicated on Figure 1 of the drawings. It is evident that this characteristic could not be carried out to 0:0, since infinite sweep width S would be required at this point. Similarly, the operating range is limited to a finite maximum distance, with a corresponding minimum sweep width. However, the voltage divider may be designed so that 0 is Proportional to over any selected range.

.An indicator I51 is coupled to the adjustable tap of the voltage divider 9 by a shaft I59, and is provided with a-scale calibrated in seconds or The shaft I59 is coupled through a difierential gear assembly I6I to a clock motor I63 or other constant speed driving means. The spider of the differential I6I is coupled to the motor 69. The clock motor I63 runs in the direction of the arrow I65, so that if the motor 69 remains stationary, the shaft I59 is driven as shown by the arrow 161, rotating the indicator I51 toward zero, and with it the tap of the voltage divider 9.

A pair of switches I69 and HI are coupled to the shaft I59. Each comprises a rotatable contact I13 and an angularly adjustable contact I15. The contact I15 is supported in a ring I11 of insulating material, which may be rotated for adjustment by means of a handwheel I19 and gearing MI. The contacts I13 and I15 of the switch I69 are connected to a bomb release mechanism, not shown. A switch I'I'2 is connected in series with the switch I69. The contacts I13 and I15 of the switch I1I are connected between a battery I83 and an electric lamp I35 or other signalling or warning .device.

A battery I93 is connected to the constant speed motor I63 through ;a switch HI and a rotary switch I81. The switch I81 comprises a rotatable contact I95 coupled .to the shaft I59 and a fixed contact I91 in the form of a sector covering an angle approximately equal to the range of motion of the movable contact of th voltage divider 9.

A switch I89, similar in construction to the :switch I81, is connected through a switch I99 .across the contacts of the relay 34. The fixed contact sector 29-: of the switch I89 forms almost a complete circle with a small gap 263 placed at, .a point such that when the shaft I59 is in the position at which the switch I99 is open, the rotatable contact .195 of the switch I81 is barely out of contact with the starting end 205 of the fixed sector I91 of switch I81. A'switch I89, identical in construction and adjustment with the switch I89, is-connected, through a switch I99, across the windings of the motor control relays '45 and 41.

The switches 2, I12, I99, 199 and I9,I are mechanically ganged together as indicated by the dash line 291, and connected to common operating means such as an arming switch or the like, not shown. In thedisarmed position, the switches I99 and I99 are closed, and the .remainderof .the switches are open, while in the armed position, switches I99 and 199 are-open and .the other switches are closed.

With the switches in the position shown in Figure '1, the transmitter I is shut o the bomb release mechanism is disconnected from the switch I69, the constant speed motor I63 is deenergized, and the motor 69 energized by means of the switches I89 and I99 to drive the shaft I59, through the differential I6I, to a position such that the rotatable contacts of the switches I89 and I89 rest upon their insulating segment 203 and 293. The mechanism is thus set at its starting position, where it remains until the arming switch is closed, closing switches 2, I12, I9I and opening the switches I99 and I99. The motors I63 and 69 remain deenergized until a signal is picked up by the receiving equipment, operating the A. V. C. rectifier 30 and closing the contacts ofthe relay 34. If the ratio (1/?) as determined by the radio equipment is not equal to the time T indicated by the indicator I51 at the starting position of the shaft I59, the

relays 45 and 41 are operated to energize the motor 69, which rotates the spider of the differential I61, moving the shaft I 59 in the direction of the arrow I61. The initial movement of the shaft I59 closes the switch I81, starting the constant speed motor I63. Thenceforth the motor I63 drives the indicator I51, through the differential I6I and the shaft I59, to indicate continuously the time from target. The voltage divider 9 is driven similarly to maintain the sweep width S at the value corresponding to the time T.

If the ratio d/o as determined by the radio equipment does not remain equal to the time T indicated by the indicator I51, the motor 69 is energized as described above, and again rotates the spider of the differential I6I, adding to or subtracting from the motion of the shaft I59 the correction necessary to make the sweep width correspond to 11/12. Thereupon the motor 69 stops, and the clock motor continues to drive the indicator I51 at a uniform rate until further correction becomes necessary. If the reflection signal fails, the relay 34 opens, disconnecting the motor 6.9, and the indicator will continue =tofunction, although no corrections will be made until it resumes.

The switch I69 is adjusted by its handwheel I19 to an angular position corresponding to the time from target at which a bomb is to be dropped. This time is determined by the speed and the altitude of the aircraft. The switch I-1-I may be adjusted similarly to .close at any desired time before .the time of release, to energize the lamp I and warn the operator or pilot that the bomb is about to be dropped. The switch -I1I may be replaced by an auxiliary contact on the switch I69, if desired.

The invention has been described as an automatic bomb release system, wherein a switch mechanism is driven by a clock or constant speed motor. The clock is set back automatically from its switch closing position by an amount equal to the distance of the target divided by the rate of approach to the target, in response to speed and distance measurements made by reflected frequency modulated radiation.

It will be apparent to those skilled in the art that various modifications of the invention can be made. For example, the carrier frequency fo or the modulation frequency Jm may be varied, rather than the sweep width S, although the latter is preferred at present. The speed and distance measuring circuits described in copending U. S. application Serial Number 524,795,

filed on March 2, 1944, by Royden C. Sanders, Jr., and entitled Radio bomb release systems may be substituted for the switched counters 33 and 35 in the system of Figure 1. A single polarized relay may replace the marginal relays 45 and 41 without substantially altering the operation of the system.

I claim as my invention:

1. An automatic bomb release system for aircraft, including means differentially responsive to the distance and the speed of a craft with respect to a target, means including a shaft for varying the extent of the response of said differentially responsive means to said distance, means for driving saidshaft normally at a constant speed, means responsive to the output of said diiferentially' responsive means to superimpose upon said constant speed drive a further rotation of said shaft to adjust said response to distance to a value such that the output of said differentially responsive means is zero, and circuit closing means responsive to the attainment by said shaft of a predetermined angular position.

2. An automatic bomb release system for aircraft, including means differentially responsive to the distance and the speed of a craft with respect to a target, means including a shaft for varying the extent of the response of said differentially responsive means to said distance, driving means coupled to said shaft to drive said shaft normally in the direction to increase the extent of the response of said differentially responsive means to said distance, means responsive to the output of said differentially responsive means to superimpose upon said normal drive a further rotation of said shaft to adjust said response to distance to a value such that the output of said differentially response means is zero, and circuit closing means responsive to the attainment by said shaft of a predetermined angular position. v

3. An automatic bomb release system for aircraft, including means difierentially responsive to the distance and the speed of a craft with respect to a target, means including a shaft for varying the extent of the response of said differentially responsive means to said distance in predetermined proportion to the reciprocal of the angle of rotation of said shaft, constant speed driving means coupled to said shaft to drive said shaft normally at a constant speed in the direction to increase the extent of the response of said difierentially responsive means to said distance, means responsive to the output of said differentially responsive means to superimpose upon said constant speed drive a further rotation of said shaft to adjust said response to distance to a value such that the output of said differentially responsive means is zero, and means responsive to the attainment by said shaft of a predetermined angular position to release a bomb.

a. An automatic bomb release system for aircraft, including means differentially responsive to the distance and the speed of a craft with respect to a target, means including a shaft for varying the extent of the response of said differentially responsive means to said distance in predetermined proportion to the reciprocal of the angle of rotation of said shaft, driving means coupled to said shaft to drive said shaft normally in the di ection to increase the extent of the response of said differentially responsive means to said distance, means responsive to the output of said differentially responsive means to superimpose upon said normal drive a further rotation of said shaft to adjust said respons to distance to a value such that the output of said differentially responsive means is maintained at a predetermined magnitude, and circuit closing means responsive to the attainment by said shaft of a predetermined angular position.

5. A bomb release system including means for radiating a frequency modulated signal to a selected target, means for receiving said frequency modulated signal after reflection by said target, means responsive to said transmitted signal and to said received signal to produce a beat signal, means responsive to said beat signal to produce an output voltage comprising a component bearing a predetermined ratio to the average frequency of said beat signal and a second component in opposition to said first component and bearing a predetermined ratio to the range of variation of frequency of said beat signal, means for controlling the range of cyclical variation in frequency of said frequency modulated signal, said last mentioned means including a shaft, means for driving said shaft normally at a constant speed, means responsive to said composite voltage to superimpose upon said constant speed drive a further rotation of said shaft to adjust said range of frequency variation to a value such that said opposed components of output voltage are equal, and circuit closing means responsive to the attaimnent .by said shaft of a predetermined angular position.

6. A bomb release system including radio transmitter means, modulator means for varying cyclically the frequency of operation of said transmitter means to provide a frequency modulated signal, means for radiating said frequency modulated signal to a selected target, means for receiving said frequency modulated signal after reflection by said target, detector means coupled to said transmitter means and to said receiving means and responsive to said transmitted signal and to said received signal to produce a beat signal having an average frequency proportional to the distance of an aircraft from said target, and a range of variation of frequency proportional to the speed of said craft with respect to said target, means responsive to said beat signal to produce a composite output voltage comprising a component bearing a predetermined ratio to said average beat frequency and a second component in opposition to said first component and bearing a predetermined ratio to said range of variation of beat frequency, means connected to said modulator means to control the range of cyclical variation in frequency of said frequency modulated signal, said last mentioned means including a shaft and a voltage divider coupled thereto and designed so that said range of variation of frequency is in predetermined proportion to the reciprocal of the angular position of said shaft, constant speed driving means coupled to said shaft through differential gearing to drive said shaft normally at a constant speed in the direction to increase said range of variation of frequency, means responsive to said composite voltage and coupled to said differential gearing to superimpose upon said constant speed drive a further rotation of said shaft to adjust said range of frequency variation to a value such that said opposed components of composite voltage are equal, and circuit closing means coupled to said shaft and responsive to the attainment thereby of a predetermined angular position.

7. A bomb release system including means for radiating a frequency modulated signal to a selected target, means for receiving said frequency modulated signal after reflection by said target, means responsive to said transmitted signal and to said received signal to produce a beat signal, means responsive to said beat signal to produce an output voltage comprising a component bearing at predetermined ratio to the average frequency of said beat signal and a second component in opposition to said first component and bearing a predetermined, ratio to the range of variation of frequency of said beat signal, means for controllin the range of cyclical variation in frequency of said frequency modulated signal, said last mentioned means including a shaft, means for normally drivin said shaft continuously, means responsive to said composites voltage to superimpose upon said normal drive. a further; rotation of said shaft to adjust said range or frequency variation to a value such that said output voltage ismaintained at a predetermined magnitude and circuit closing means responsive to the. attainment by said shaft of a predeter" mined angular position.

8. An automatic bomb. release system for aircraft, including means differentially responsive to the distance and the speed of a craft with respect to a target, means including a shaft for varying the extent of the response of said differentially responsive means to said distance, means for driving said shaft. normally to continuously change the extent of the response of said differentially responsive means to said distance,

means responsive t the output of said differentially responsive means to superimpose upon said normal drive a further rotation of said shaft to adjust said response to distance to a value such that the output of said difi'erentially responsive means is maintained at a predetermined magnitude and circuit closing means responsive to the attainment bysai'd shaft of a predetermined angular position.

9. An automatic bomb release system for aircraft, including means differentially responsive to the distance and the speed of a craft with respect to a target, means including a shaft for varying the extent of the response of. said difierentially responsive means to said distance, means for normally driving. said shaft continuously, means responsive to the output of said differentially responsive means to superimpose upon said normal continuous drive a further rotation of said shaft to adjust said response to distance to a value such that the output of said differentially responsive means. is maintained at a predetermined magnitude, and circuit closing means responsive to the attainment by said shaft of a predetermined angular position.

ROYDEN C. SANDERS, Ja.

REFERENCES CITED- UNITED STATES PATENTS Name Date Procofieff-Seversky Jan. 7, 1936 Number 

